Study of Long-Term Pavement Performance (LTPP): Pavement Deflections

Chapter 7. Suggested Computed Parameters and FWD Testing
Protocols

The Initial Working Data Files for the Project

As mentioned near the beginning of this report, the originally
provided database files were reorganized to facilitate further
analyses. Specifically, day files were created under directories
organized by region and test section; these files consisted of all
the data for each date of test at each LTPP section. These day
files were changed in accord with the findings of this study to
include the deletions, changes, and notes reported above. All of
these files were written to a computed parameter data CD, which
consisted of normalized and averaged FWD load-deflection data, with
data errors and anomalies identified, changed, or deleted from the
files as appropriate.

The Modified (Computed Parameter) Individual Data
Files

As mentioned previously, each day file was identified by a
six-character date (yymmdd). After each date, in the seventh
character of the file name, the letter “n” or “m” was employed. The
letter “n” stands for normalized, while “m” stands for modified
after normalization. Finally, each normalized and/or modified day
file has the extension “*.txt” since these are comma-delimited
files. Occasionally, “u” or “x” was used instead of the usual “n”
or “m.” These two letters denote that a note only is present,
with no data. Such notations are further explained below.

The “n” files denote those day files that have been screened for
inconsistent basin type load-deflection errors. They have been
normalized to the target load levels for each drop height, while
each line or record represents the average of the deflection
readings for a given test point and drop height. The field for drop
sequence number has been replaced with a field indicating the
number of drops (1, 2, 3, or 4) used to create the averages. The
“n” files have no other changes or notes attached. The “m” files
denote that some additional change or changes have taken place. In
all these cases, there is a note in the last record of the
“m” file that explains the changes or deletions (in some cases) or
warns the user of potential problems (in other cases). These
pre-autumn 1998 data files presently exist on a single data CD
(approximately 100 megabytes).

However, it was decided that these files would not be
incorporated into the LTPP database. Therefore, no computed
parameters of pavement deflections exist at this time in the LTPP
database. It is, however, still possible that the “n” files could
eventually be consolidated into database tables so that they are
available for general use.

FWD Test Protocol Recommendations

Based on the findings of this report and other considerations,
the following LTPP FWD testing procedure changes were recommended
in 1999:

Install the Dynatest Edition 25 Field Program, in lieu of the
currently employed Edition 20.

Utilize all nine available deflection sensors, in lieu of the
currently employed seven sensors.

Place sensors 1 through 9 permanently at the following
positions: 0, 203, 305, 610, 914, 1219, 1524, 1829, and –305 mm,
respectively. These positions will permit all types of tests with
no maneuvering of sensors. The corresponding U.S. customary sensor
positions, in inches, are: 0, 8, 12, 18, 24, 36, 48, 60, and –12,
respectively.

With a simple software change to Edition 25, request Dynatest
to allow for the recording of both the infrared (IR)
pavement surface temperature at the time the F1 key is pressed, and
also at the time a test sequence ends; the current procedure only
records the latter measurement. One of the many available fields in
the comma-delimited “Station” line of Edition 25 can be used for
the extra temperature value (along with the usual time-of-departure
IR temperature reading).

Continue utilizing three seating drops, as previously, at drop
height 3; however, record the peaks of these three seating drops
for potential use in analyzing the pavement’s hardening or
softening properties, which may be related to pavement
performance.

It is adequate to utilize only three drop heights for
all types of bound-layer tests, whether PCC or AC, and for all
types of LTPP testing, whether General Pavement Studies (GPS),
Specific Payment Studies (SPS), or the Seasonal Monitoring Program
(SMP). These drop heights should be 2, 3, and 4, or approximately
9, 12, and 16 kips, respectively.

It is adequate to utilize only three drops per drop
height for all types of bound-layer tests, whether PCC or AC, and
for all types of LTPP testing, whether GPS, SPS, or SMP.

It is adequate to store only one full load-deflection
time history for each test point, for all types of bound layer
tests, whether PCC or AC, and for all types of LTPP testing,
whether GPS, SPS, or SMP. If it is not possible to record more than
one full time history per test point, this full time history should
be the last drop at the highest utilized drop height.

Utilize the same spacing between test points as is currently
used for the various types of experiments (GPS, SPS, and SMP).

Do not eliminate or delete FWD operator comments from the level
E database.

Make sure both IR temperatures (at factory calibration
settings) and manual temperatures are monitored according to the
original protocols, and that the temperature measuring equipment is
operating properly. Reemphasize the importance of correct protocol
in measuring indepth temperatures.

Furnish the regions with the transformed basin or SLIC
procedure software for all types of data storage files. This
screening tool should help correct or eliminate data errors well
before they reach level E in the database, whether these
data errors may be in the form of misplaced sensors, faulty
sensors, or sensor holders.

Emphasize the importance of checking sensor spacings and the
stability of the sensor holders, magnets, and other equipment
(including the center sensor) prior to testing at each LTPP test
section.

All other current FWD testing and QA/QC protocols and
procedures—especially those involving accuracy of actual sensor
positions and the correct method of conducting pavement temperature
measurements—should remain the same.

Further, an analysis was conducted to develop a procedure to
determine if more frequent LTPP testing should be conducted as a
pavement ages and exhibits more distress. The approach was to use a
representative measure of the deflection basin, adjust this measure
for pavement temperature, and determine whether definitive trends
in the selected deflection measure exist.

The following specific analysis approach was used:

After a review of various deflection basin measures, it was
decided to use the AREA measure to represent the overall
characteristics of the deflection basin. The AREA measure is a
calculation of the normalized area of a deflection basin.
Facilitating the use of the AREA measure was the fact that a
temperature adjustment procedure particularly pertinent to the AREA
measure was available. The AREA, using U.S. customary units, has
traditionally been defined as shown by the equation in figure
5:

A list was developed of GPS–1 and GPS–2 test sections that had
at least four different times (nonseasonal) of deflection testing
between 1989 and 1998. Ten of these test sections were randomly
selected for further analysis.

The AREA values were determined for each F3 test point
(wheelpath testing at 7.6-m (25-ft) test intervals).

The average middepth temperatures during the approximately 1
hour of wheelpath testing were established, for each test visit and
for each of the 10 sections.

The basin adjustment factor (TAF) for the AREA parameter was
established using the procedure described in a previous FHWA study,
Temperature Predictions and Adjustment Factors for Asphalt
Pavement.(2) Although
the TAF is a function of the AC layer thickness and the latitude of
the test section, it typically ranges in almost a linear manner
from about 0.90 at 0 ºC (32 ºF) middepth temperature to about
1.1 at 40 ºC (104 ºF) middepth temperature, using the reference
middepth temperature of 20 ºC (68 ºF). The temperature adjusted
AREA is then given by the equation in figure 6:

Figure 6. Equation. AREA (@ 20
ºC).

All AREA values determined in step 3 were adjusted to account
for the middepth pavement temperature as shown in step 5. The
adjusted AREA values are listed in table 8.

A review of the results shown in table 8 indicates no definitive
trends in average AREA values with time for the ten sections used
in the analysis. Therefore, no specific recommendations can be
provided at this time for considering changes in the frequency of
deflection testing. Also, no acceptable procedures presently exist
to adjust the maximum deflections for temperature at the time of
test, although this may be possible in the near future. Finally, it
may be necessary to include sections exhibiting significant
cracking and/or rutting distress in the study sample, since it is
possible that the deflections do not change appreciably until some
pavement distress ensues.

The recommendations shown above were reported in 1998. Most (but
not all) of these recommendations have since been implemented in
connection with FWD upgrades, annual equipment servicing, and other
events, and the FWD deflection testing protocols have been changed
accordingly, where appropriate.